Hi Benoit,
In general you need to upgrade the agents as soon as possible.
Any kind of changes in the message structure between the agent and the broker could cause problems.
However for 10.1 -> 10.3, I do not believe there have been major changes to the messages (there may be a few minor changes).
In summary, for these two versions, upgrading is not critical but should be done as soon as possible.
Also if you need supports assistance, then upgrading will be required.
Best Regards,
Ricky
DevTest Support